The production of polyurethan foams, also commonly referred to as polyurethane foams, has been known for some time. In general, methods of production involve reacting a polyol, e.g., di- or polyfunctional hydroxy compound, such as polyhydroxyl containing polyethers, with di- or polyfunctional isocyanates in the presence of a foaming agent, optionally including other additives such as catalysts, cell-regulating surfactants, stabilizers, and the like. As is well known, foaming is usually accomplished by including water in the reaction system, the reaction between isocyanate and water providing carbon dioxide for foaming. Other foaming agents can be used, however, such as solid chemical or liquid "blowing agents".
The polyhydroxyl containing polyethers presently in common use in the production of flexible polyurethane foams are prepared by condensing one or more alkylene oxides on compounds having at least two active hydrogen atoms, such as, for instance, glycols, triols, tetrols, amine or mixtures thereof.
The alkylene oxides most commonly used are ethylene oxide and propylene oxide, either individually or in admixture with each other. Where mixtures of the two are employed, it is generally accepted that the ethylene oxide content should not exceed 45% by weight of the total mixture, otherwise the surface tension of the polyol as well as the resulting polymer will be much too high and will hinder the expansion or "growth" of the polyurethane in the presence of silicone surfactants. However, the use of polyethers having a high ethylene oxide content is necessary in the production of flexible polyurethan foams having low load-bearing capacities.
In order to overcome such drawbacks, it has been proposed in French Pat. No. 2,129,823, that the workability of a polyol having a high content of ethylene oxide, e.g., 58-77%, can be improved by mixing it with another polyol of very low ethylene oxide content, e.g., 4-15%. The resulting system exhibits a sufficiently low surface tension to be readily processable with commonly used silicone surfactants.
According to the aforementioned French patent, the amount of primary hydroxyl groups must be 35-44 percent hydroxyl groups present in the polyol of high ethylene oxide content and 35-60 percent primary hydroxyl groups present in the polyol of low ethylene oxide content. Moreover, the ratio of weight percent value of ethylene oxide to the percent value of primary hydroxyl groups based on the total number of hydroxyl groups present must be between 1.3 and 1.7.
A shortcoming in the process of the French patent is that two different free polyols are required in the reaction mixture and these are not compatible with each other. Another disadvantage is that as a result of using polyols having a relatively high proportion of primary hydroxyl groups, i.e. anywhere from 35 to 60 primary hydroxyls per 100 hydroxyl groups present only an amine-type catalyst can be employed in the system. This hinders the attainment of a proper balance between the polymerization reaction, i.e. the isocyanate-hydroxyl reaction, and the expansion or foaming reaction, i.e., the isocyanate-water reaction, which in continuous bulk polymerization methods is carried out with the use of a pair of catalyst: a metallic catalyst, e.g., stannous octoate, tin dibutyl dilaurate, and the like, in combination with an amine catalyst, e.g., tertiary amine.
A principal object of the present invention is to provide a method for producing polyurethane foams by the use of only a single free polyhydroxyl containing compound in the reaction mixture and a relatively low content of primary hydroxyl groups, thus avoiding the above-mentioned shortcomings connected with the process of the French patent.
This object and others which will be apparent from the following description, are accomplished by the present invention.